PETpla.net Insider 11 / 2016

INSPECTION 26 PET planet Insider Vol. 17 No. 11/16 www.petpla.net measurements externally coated bot- tles show similar oxygen ingress rates as non-coated bottles, because the outside partial pressure is in equilib- rium with air and considerably higher than the inside partial pressure. This is the case until a new equilibrium within the polymer is established. After an equilibrium state between matrix oxygen in the PET and the product is reached a linear increase of oxygen ingress over time can be measured. As long as the satura- tion level is not reached the increase of oxygen is expected to follow a straight line. The slope of the curve gives the oxygen ingress in ppb/day. Also the total oxygen can be calcu- lated from the slope of the curve. Knowing the maximum allowed concentration of O 2 in the product, 1mg/l in the case of beer, and dividing it by the slope of oxygen increase the theoretical shelf-life can be calculated. The externally coated bottle shows a reduced oxygen permeation rate but a shelf-life of approx. 53 days is not sufficient since usually a shelf-life of at least six months is demanded. This proves that even the externally coated PET bottle would need improvement using oxygen scavengers in the mate- rial to protect the product. Test 2: Oxygen scavengers in PET bottles In a second test different amounts of oxygen scavenger were used in the bottle material. Desorption of oxygen from the PET bottle wall into the product is one of the main disadvan- tages of externally coated bottles. The matrix oxygen dissolved in the PET bottle can be removed by adding an oxygen scavenger into the PET mate- rial. Measurements in a non-coated bottle containing 2% oxygen scav- enger were compared to an identical bottle without scavenger. Further- more, LC2 externally coated PET bot- tles with either 1% and 0.5% oxygen scavenger were tested. In the non-coated PET bottle containing 2% oxygen scavenger the matrix oxygen is removed by the scavenger and therefore the meas- urements show no initial increase in oxygen at the beginning (fig. 4). After eight days though, a strong increase in oxygen concentration is detected. This indicates that the scavenger is being consumed. After 20 days, the graph of the PET bottle with scav- enger shows an identical slope to the non-coated PET bottle, as the scavenger is completely consumed by then. The LC2 external coated bottle without scavenger shows a strong oxygen increase over the first 48 hours as described in test 1. When the equilibrium between matrix oxygen and dissolved oxygen in the liquid is reached the graph again dis- plays the linear slope. After 40 days the curve of the LC2 external coated bottle without scavenger intersects the graph of the non-coated bottle with 2% scavenger. The oxygen ingress in both of these bottle types is too high and the maximum oxygen pick up of 1,000ppb at 30°C is reached before a 180 days period. These bottles would not guarantee a long enough shelf-life of products. Bottles with a combination of LC2 external coating and 1% or 0.5% scavenger were also measured. The scavenger is located in the inner PET layer and protected from ambient oxygen by the external barrier coating. The inner-layer scavenger reacts with oxygen molecules permeating through the outside coating and is also avail- able to decrease oxygen in head- space and liquid, which is integrated during the filling process. The combi- nation of external coating and scaven- ger results in a significantly reduced oxygen ingress rate, below 1,000ppb over six months. As the scavenger is protected by the outer barrier layer from ambient oxygen and only reacts with molecules permeating through the barrier its lifetime is longer, and it Fig. 4: Oxygen ingress into different treated PET bottles of the same type; (A) non- coated PET bottle with 2% scavenger, (B) LC2 coated PET bottle with no scavenger, (C) LC2 coated PET bottle with 1% scavenger, (D) LC2 coated PET bottle with 0.5% scavenger Fig. 5: Comparison of oxygen ingress into an Actis coated bottle and the respective non-coated reference bottle